This proposal is concerned with the mechanism of an important and interesting phenomenon seen in the inheritance of mitochondrial genes in yeast (Saccharomyces cerevisiae). This phenomenon, called polarity, is characterized by the net loss of mitochondrial alleles contributed by one parent from zygotes which are heterozygous at the mitochondrial omega locus and at the newly-discovered varl locus. Similar conversion events have been identified in other systems but those systems are not currently appropriate for a detailed analysis of the mechanism of the process. Our previous work has shown that polarity is seen in cells heterozygous for insertions (or deletions) of mtDNA, and involves the preferential conversion of the shorter allele to the longer one during recombination between mtDNA molecules. Taking into account the new results obtained in the last three years, we have modified our original hypothesis and propose a new molecular mechanism for polarity, in which heteroduplex DNA segments are formed in the region of insertions, with a single-stranded loop opposite a specific DNA sequence. This heteroduplex is recognized by an enzyme system and the specific sequences preferentially nicked and excised. Gap-filling DNA synthesis forms a new insertion region of the strand which originally lacked one. Our approach to studying the mechanism of polarity at omega rests on a detailed physical and genetic dissection of the relevant genome segment including the isolation of new point and deletion mutants there. This functional analysis of the region will be extended by selective DNA sequencing studies. We will also ask whether any of the many other strain-specific insertions undergo unidirectional gene conversion and, if so, whether that process affects the transmission of nearby point mutations. Comparison of insertions capable and incapable of gene conversion will enable us to probe the structural and sequence requirements of that process. We believe that yeast mitochondria are an excellent system for investigating the general phenomenon of recombination involving insertions, duplications, deletions and frameshifts.